The present invention relates to a power semiconductor device and a power conversion device using the power semiconductor device.
An insulated gate bipolar transistor (which will be abbreviated to the IGBT, hereinafter) is an switching element which controls a current flowing between a collector electrode and an emitter electrode with use of a voltage applied to a gate electrode. The IGBT has a controllable power range of from tens of watts to hundreds of thousands of watts, and also has a switching frequency as broad as from tens of Hertz to over a hundred Hertz. Taking advantage of this feature, the IGBT is widely used from a domestic small-power appliance such as an air conditioner or a microwave oven to a large-power apparatus such as an inverter in railway or an ironworks.
In order to attain a high efficiency of a power conversion device or the like to which the IGBT is applied, the IGBT is required to reduce its loss. To this end, various measures have been taken. The loss includes a conduction loss, a turn-on loss, and a turn-off loss. A voltage generated between the collector electrode and the emitter electrode in the ON state of the IGBT is called an ON state voltage, which is used as an index for making the ON state voltage proportional to the conduction loss. Thus it becomes important in the IGBT to reduce the ON state voltage, the turn-on loss, and the turn-off loss.
FIG. 11 shows a cross-sectional structure of an IGBT 20 as a prior art cited in JP-A-2000-307116, Paragraph (0056). FIG. 11, reference numeral 500 denotes a collector electrode, numeral 100 denotes a p layer contacted with the collector electrode 500 with a low contact resistance, 112 denotes an n layer having a carrier concentration lower than the p layer 100, 110 denotes an n− drift layer having a carrier concentration lower than the n layer 112, 120 denotes a channel p layer, 121 denotes a p+ layer, 125 denotes a floating p layer, 130 denotes an n+ layer, 600 denotes an emitter electrode contacted with the p+ layer 121 and with the n+ layer 130 with a low contact resistance, 300 denotes a gate insulated film, 200 denotes a gate electrode, 401 denotes an insulating film, 501 denotes a collector terminal, 601 denotes an emitter terminal, and 201 denotes a gate terminal.
The IGBT of FIG. 11 corresponds to a general trench insulated gate type IGBT but which has a reduced number of the emitter electrodes 600 to shorten a gate width, thereby reducing a saturation current, suppressing a short-circuited current, and increasing a breakdown resistance. In the IGBT 20 of FIG. 11, further, the number of the emitter electrodes 600 is reduced and the floating p layer 125 is introduced, so that part of a Hall current flows through the floating p layer 125 into an emitter. As a result, a Hall concentration in the vicinity of the emitter is increased, the resistance is decreased, thus reducing the ON state voltage.
Many devices as improved thyristors having ON state voltages lower than the IGBT but hard in switch control are reported. A MOS-gated emitter switched thyristor (referred to merely as EST, hereinafter), a MOS controlled thyristor (referred to merely as MCT, hereinafter), and a base resistance controlled thyristor (referred to merely as BRT, hereinafter) are widely known. In recent years, an insulated gate turn-off thyristor (referred to merely as IGTO, hereinafter) is proposed.
FIG. 12 shows an IGTO 30 having a current saturation capability as cited in JP-A-2000-311998, Paragraphs (0013) to (0016), FIG. 2. The IGTO 30, as shown in FIG. 12, has a structure of a combination of a thyristor and an IGBT. In the IGTO, a device base has an anode electrode 72 having an anode terminal 61, a P+ layer 71, and an N− drift layer 70. A deep trench gate has a gate terminal 60 having a gate electrode 73, a conductive material 74, an oxide walls 75, 76, 77. The deep trench gate is provided in the device base. Also provided on the trench gate are a P− base 69, a P+ layer 68, an upper-side angled N+ layer 67, and a cathode electrode 66 having a cathode terminal 62, which are disposed around the trench gate. As a result, the IGBT is formed. On the other hand, the thyristor has a P− base 65, an N+ layer 64, and an oxide layer 63. A shallowly-doped P channel 78 are provided under the trench gate. In the IGTO 30, the thyristor is turned on by turning on the IGBT, so that the operation of the thyristor achieves the ON state voltage.
However, the IGBT 20 shown in FIG. 11 (JP-A-2000-307116, Paragraph (0056)) or the IGTO 30 shown in FIG. 12 (JP-A-2000-311998, Paragraphs (0013) to (0016), FIG. 2) has a problem which follows. It has been found in the IGTO 30 of FIG. 11 that, as the width of the floating p layer 125 is increased to reduce the saturation current, the ON state voltage increases. The inventors of this application has located through detailed analysis of their calculation that the major cause of the problem comes from the fact that electric charges within the IGBT are unevenly distributed. FIG. 13 shows a calculation result of a charge concentration distribution (linear scale) when charges pass through an E-E′ sectional plane in FIG. 11. Since a charge concentration is reduced in the vicinity of the emitter, this emitter region has a high resistance, thus increasing the ON state voltage. FIG. 14 shows calculation results of charge concentration distributions (logarithmic scale) when charges between an F-F′ plane in the vicinity of the emitter in FIG. 11, a G-G′ plane in the center portion between the emitter and the collector, and between an H-H′ plane in the vicinity of a collector, respectively. In the vicinity of the emitter, a region of the floating layer away from the emitter electrode has an extremely-reduced charge concentration. Since the saturation current is reduced, it is required to improve uneven charges for the purpose of spreading the width of the floating P layer.
In the IGTO 30 of FIG. 12, when two thyristors are formed between the IGBTs as an example, a lower forward voltage drop is provided and the current saturation function is made less effective, which is disclosed in JP-A-2000-311998, Paragraph (0034). In other words, the JP-A-2000-311998 indicates even in the IGTO 30 of FIG. 12 that the reduction of the ON state voltage is inconsistent with the increase of the breakdown resistance based on the current saturation function.